In recent years, many image pickup apparatuses such as digital single lens reflex cameras employ image sensors (referred to as CMOS sensors, hereafter) using CMOS (Complementary Metal Oxide Semiconductor).
A CMOS sensor has a plurality of pixels that are arranged in two dimensions. In an image pickup optical system that uses this kind of CMOS sensor, for example, a mechanical shutter may be used as a mechanical shielding member to control an exposure time.
When taking a still image, the mechanical shutter shades the pixels and all the pixels are reset under the shaded condition. Then, the mechanical shutter opens to expose the CMOS sensor so as to take suitable exposure. After a predetermined time passes, the mechanical shutter closes, and then, a signal is read to obtain a still image.
However, driving speed (i.e., shutter speed) of the mechanical shutter is limited. Accordingly, an electronic shutter is used instead of the mechanical shutter in order to increase shutter speed, in general.
Here, an example of operation of the electronic shutter will be described. It should be noted that a mechanical shutter is opened so that the CMOS sensor is always exposed when using the electronic shutter.
First, a scan to remove an unnecessary charge stored in each pixel, i.e., a reset scan, is executed for each pixel or each line of pixels. Then, when a predetermined time passes for each pixel or each line of pixels, a scan to read a stored electric charge is executed. This achieves a function of the electronic shutter. Such an electronic shutter is referred to as a “rolling electronic shutter”, hereafter.
FIG. 11 is a view schematically showing an example of configuration of an image pickup unit that uses a conventional CMOS sensor. The CMOS sensor concerned is scanned by an X-Y address scanning method. An image pickup apparatus has the image pickup unit shown in the figure. FIG. 12 is a view showing an example of a circuit arrangement of each pixel of the CMOS sensor shown in FIG. 11. FIG. 12 shows one pixel among the pixels shown in FIG. 11.
It should be noted that FIG. 11 shows the CMOS sensor that has nine pixels 1104a in three lines and three columns (3-by-3 matrix) arrangement in order to simplify a description. However, an actual CMOS sensor has an extremely large number of pixels (for example, millions of pixels) that are arranged in two dimensions.
In FIG. 11, the image pickup unit that uses the CMOS sensor shown in the figure is provided with a vertical shift register (a vertical scanning circuit) 1101, readout circuits 1102 and 1110, horizontal shift registers (horizontal scanning circuits) 1106 and 1109, output amplifiers 1105 and 1111, a pixel unit 1104, constant current sources 1107a through 1107c, and selection switches 1103a through 1103c. And the pixel unit 1104 is provided with the pixels 1104a of 3-line by 3-column.
As shown in FIG. 11, in the pixel unit 1104, a signal output line 1108a is connected to three pixels 1104a on a first column, and a signal output line 1108b is connected to three pixels 1104a on a second column. Similarly, a signal output line 1108c is connected to three pixels 1104a on a third column.
Constant current sources 1107a through 1107c are connected to these output signal lines 1108a through 1108c, respectively. The output signal lines 1108a and 1108c are connected to the readout circuit 1102. The output signal line 1108b is connected to the readout circuit 1110.
As shown in FIG. 11, the readout circuit 1102 is connected to the output amplifier 1105 via the selection switches 1103a and 1103c, and the readout circuit 1110 is connected to the output amplifier 1111 via the selection switch 1103b. And the selection switches 1103a and 1103c are controlled by the horizontal shift register 1106, and the selection switch 1103b is controlled by the horizontal shift register 1109.
The above-mentioned vertical shift register 1101 gives a transfer pulse signal PTX, a reset pulse signal PRES, and selection pulse signal PSEL to the pixel unit 1104. Accordingly, the pixel unit 1104 outputs an electric charge as an output voltage as mentioned later.
It should be noted that a scan line defined by the pixels 1104a in the first line is referred to as a first scan line, here. That is, the scan line defined by the pixels 1104a of the n-th line (n is integer equal to or larger than 1; n=1, 2, or 3 in the shown example) is referred to as the n-th scan line.
The transfer pulse signal, the reset pulse signal, and the selection pulse signal that the vertical shift register 1101 applies to the n-th scan line are expressed by PTX(n), PRES(n), and PSEL(n), respectively.
In FIG. 12, the pixel 1104a has a photodiode (PD) 1201, a transfer switch 1202, a reset switch 1203, a storage region (floating diffusion: FD) 1204, a MOS amplifier 1205, and a selection switch 1206.
Incident light to the pixel 1104a is converted into an electric charge by the PD 1201. And the transfer pulse signal PTX turns the transfer switch 1202 ON, and the electric charge converted by the PD 1201 is transmitted to the FD 1204. On the other hand, the reset pulse signal PRES turns the reset switch 1203 ON, and the electric charge stored in the FD 1204 is removed. The MOS amplifier 1205 is an amplifier that functions as a source follower. The selection switch 1206 is turned ON by the selection pulse signal PSEL, and selects the pixel from which a stored electric charge is read.
What is called a floating diffusion amplifier comprises the FD 1204, the MOS amplifier 1205, and the constant current sources 1107a through 1107c mentioned later. As mentioned above, the electric charge converted by the PD 1201 of the pixel that is selected by the selection switch 1206 appears in a signal output line 1207 as an output voltage of the MOS amplifier 1205.
It should be noted that the signal output line 1207 shown in FIG. 12 shows either of the signal output lines 1108a through 1108c shown in FIG. 11.
The constant current sources 1107a through 1107c become loads on the MOS amplifier 1205, respectively, and the output voltages from the pixels 1104a on the first column are given to the readout circuit 1102. Similarly, the output voltages from the pixels 1104a on the third column are given to the readout circuit 1102. The output voltages from the pixels 1104a on the second column are given to the readout circuit 1110.
As mentioned above, the selection switches 1103a and 1103c are driven and controlled by the horizontal shift register 1106. The output voltages read by the readout circuit 1102 are driven and controlled by the selection switches 1103a and 1103c, and are outputted via the output amplifier 1105.
The selection switch 1103b is driven and controlled by the horizontal shift register 1109. The output voltages read by the readout circuit 1110 are driven and controlled by the selection switch 1103b, and are outputted via the output amplifier 1111.
FIG. 13 is a view showing an operating sequence when the CMOS image pickup unit shown in FIG. 11 makes a rolling electronic shutter operate. It should be noted that FIG. 13 shows a driving control for selected four lines from the n-th scan line to the (n+3)th scan line in order to simplify the description.
First, in the n-th scan line, the reset pulse signal PRES(n) and the transfer pulse signal PTX(n) are applied to the pixel unit 1104 of the CMOS sensor during a period between the time t32 and the time t31. This turns ON the transfer switch 1202 and the reset switch 1203 shown in FIG. 12. The reset operation is executed to remove the unnecessary electric charge stored in the PD 1201 and the FD 1204 in the pixel 1104a of the n-th scan line (the n-th line).
Then, the transfer switch 1202 turns OFF at the time t32, and the storage operation to store an electric charge generated in the PD 1201 is started. Next, at the time t34, the transfer pulse signal PTX (n) is applied to the pixel unit 1104, and the transfer switch 1202 is set to ON. This causes a transfer operation to transfer the electric charge of the PD 1201 to the FD 1204.
It should be noted that the reset switch 1203 must turn OFF before the above-mentioned transfer operation. In the drive controlling shown in FIG. 13, the reset switch 1203 and the transfer switch 1202 turn OFF simultaneously at the time t32.
As mentioned above, the time period from the end of reset operation at the time t32 to the end of transfer at the time t35 becomes storage time.
The selection pulse signal PSEL(n) is applied to the pixel unit 1104 after finishing the transfer operation for the electric charge stored in the pixel 1104a of the n-th line. This turns the selection switch 1206 ON. When the selection switch 1206 turns ON, the electric charge stored in the FD 1204 is converted into a voltage, and the voltage is outputted to the readout circuits 1102 and 1110 as the output voltage.
The voltage signals temporarily held by the readout circuits 1102 and 1110 are sequentially outputted by the horizontal shift registers 1106 and 1109 from the time t36. A time period from a start of transfer at the time T34 to an end of readout at the time t37 is defined as T3read, and a time period from the time t31 to the time t33 is defined as T3wait.
It should be noted that a time period from a start of transfer to an end of readout also becomes the T3read in another scan line, and a time period from a start of reset for a certain scan line to a start of reset for a next scan line also becomes T3wait.
The drive method of the above-mentioned rolling electronic shutter enables to take a still image. As shown in FIG. 13, the transfer pulse signal PTX(n+3) and the selection pulse signal PSEL(n+3) are applied at the second time following the reset. Then, the transfer pulse signals PTX(n) through PTX(n+3) and the selection pulse signals PSEL(n) through PSEL(n+3) are repeatedly applied without resetting. Accordingly, the signal for moving images of which the readout interval is the storage time Tint can be acquired.
There is what is called a batch electronic shutter operation MOS type image sensor that executes a reset operation and a read operation by one operation. FIG. 14 is a view showing an operating sequence of the batch electronic shutter operation MOS type image sensor.
In FIG. 14, the reset actions for all the scan lines are simultaneously executed between the time t41 and the time t42. And the transfer operations are also simultaneously executed between the time t43 and the time t44. In the operation of the batch electronic shutter, the storage time is a time period between the time t42 and the time t44 for all the scan lines (for example, see PTL 1).
Incidentally, the CMOS sensor is required to increase the pixel number, the shooting speed, and the ISO (sensitivity). Such a CMOS sensor with a large number of pixels is extremely useful to take a high definition still image. The pixel number is over 10 million pixels in recent years.
On the other hand, the pixel number required to take a moving image is about 300 thousand pixels in general and is about 2 million pixels when conforming to the full standard of high-definition TV, which is fewer than the pixel number required to take a still image. A frame rate is about 30 frames per second or 60 frames per second.
When taking a moving image using the CMOS sensor with a large number of pixels that is designed to take a still image, it is common to perform a process for thinning out pixels or a process for summing pixels from a point of view of a pixel number and a frame rate.
As a method to increase shooting speed, there is a known technique to arrange charge-storage elements (pixel memories) around the PD corresponding to each pixel and to store electric charges into these charge-storage elements when storing (for example, see PTL 2).